Use of the Distribution Network Operator (DNO) fuse for short-circuit protection of the surge protective device (SPD)

With recent industry discussions on surge protection devices (SPDs) and overcurrent protection devices (OCPDs), the ENA and BEAMA have published a joint statement to provide clarity on the use of the Distribution Network Operator (DNO) fuse for short-circuit protection of the SPD.

Overcurrent protective device (OCPD) 2 in BS 7671 Fig 534.5 may be omitted and OCPD 1 e.g. the DNO cut-out fuse used for the short-circuit protection of the SPD where all of the following apply:

• The SPD is in a household or similar installation.
• The SPD is installed either inside a single-phase consumer unit (CU) conforming to BS EN (IEC) 61439-3, or inside an enclosure together with a switch-disconnector supplied as a composite unit conforming to BS EN (IEC) 60947-3. Both these products shall have a rated conditional short-circuit current of 16 kA.
This conditional rating is qualified using a 100 A BS 88-3 (formerly BS 1361) fuse which also covers 60 A and 80 A fuse ratings. BS 1361 type II and BS 88-3 fuse-link key performance characteristics are identical therefore, either is acceptable.
• The SPD conforms to BS EN 61643-11.
• The SPD manufacturer’s instructions state that OCPD 2 can be omitted and specify the required OCPD 1 characteristics.
• The SPD does not require withdrawal of the DNO cut-out fuse for its replacement or maintenance.

This position is in line with the safety aspects of the DNO cut-out not providing functional protection but making it clear that it can be relied upon in extreme conditions to perform a short duration fault clearing role.

A 16kA conditional rating accounts for unknown future network changes and increased fault levels over the lifetime of the installation.

The Electricity Safety, Quality and Continuity Regulations (ESQCR) is not breached: Reg 25 regarding making or altering a connection is not compromised by simply relying on the protective characteristics of DNO equipment. The DNO OCPD1 remains to be owned and maintained by the DNO and is not changed by the installer.

Find out more here.

beama.org.uk